1) you can use the plugin, there are also topics on this site about saving field groups to a custom location. I generally build this into my plugins to force saving field groups used by the plugin into a folder on in the plugin and load field groups from there.
2) I have thought about this as well. There are filters that are run on field groups before and after the json saving/loading. It is very likely that the private flag could be turned on and off by your plugin using these filters and checking some env setting when they run, but I don’t know this for sure.
